1. Field of Invention
The present invention relates to an amplifier for source driver. More particularly, the present invention relates to a buffer amplifier for source driver.
2. Description of Related Art
Nowadays, flat panel displays, for example, such as LCD, have been used widely. Along with the improvement of the semiconductor technology, LCD is known to have such advantages as lower power consumption, smaller size and lighter weight, higher resolution, higher color saturation, and longer lifespan. Thus, LCD is widely applied to electronic products related to our daily lives, such as the use of LCD screens in laptop computers or desktop computers and LCD TVs. In which, the source driver is a critical component of a LCD.
Furthermore, a source driver is further divided into a digital circuit portion and an analog circuit portion. A buffer amplifier, for example, such as an output buffer amplifier or a reference voltage generator, is usually used in the analog circuit portion. Generally, a buffer amplifier, for example, used at an output stage for driving a LCD panel, requires rail-to-rail. In other words, the so-called rail-to-rail input is to mean that the range of the input signal can be from the higher power supply voltage level (referred to as VDD) of the IC to the lower power supply voltage level (referred to as VSS) of the IC. Similarly, the rail-to-rail output is to mean that the range of the output signal can be from VDD to VSS.
A conventional amplifier circuit is disclosed in Taiwan Patent Publication No. 455833 by Sharp Corp., in which a plurality of switches are used for performing DC offset cancellation. However, a larger chip layout area will be consumed by the footprint of the amplifier structure of the aforementioned patent when used as the output buffer. Moreover, the DC offset of the LCD panel to which the amplifier is applied is usually within an acceptable range.
In addition, another conventional amplifier circuit is disclosed in Taiwan Patent Publication No. 340995 by Sharp Corp., in which the amplifier structure can have a dynamic input range. However, the gate of the output NMOSFET (N-type metal oxide semiconductor field effect transistor) in the aforementioned circuit is coupled to a constant bias, which results in an inequality between the charge rate and the discharge rate of the output stage. If the aforementioned circuit is used as a reference voltage generator in a source driver inside a plurality of amplifiers A101 in the circuit shown in FIG. 1, inaccurate voltage levels may be resulted. The aforementioned circuit is formed of the amplifiers A101 and a plurality of resistors R101 as shown in FIG. 1. Moreover, DC power is consumed regularly because the gate of the output NMOS in the circuit is coupled to a constant bias.